The present invention relates to a method for the production of acetic acid with reduced formation of carbonyl impurities, especially acetaldehyde and acetone, by carbonylation of methanol and/or dimethyl ether and/or methyl acetate in the presence of a rhodium catalyst and a methyl halide.
Carbonylation processes in the presence of rhodium catalysts are known and described, for example in U.S. Pat. No. 3,768,329.
Carbonylation processes in the presence of ruthenium and osmium catalysts are known from GB Patent Nos. 1,234,641 and 2,029,409.
A carbonylation process combining rhodium as a carbonylation catalyst and ruthenium and/or osmium as a promoter for increasing reaction rate at specified conditions is, furthermore, known from EP 728,727.
The general object of this invention is to provide a process for carbonylation of methanol or reactive derivatives thereof with suppressed formation of undesired by-products, causing major separation problems, such as acetaldehyde and acetone, whereas certain by-products, which are easily separated and even may represent a valuable coproduct, such as propanoic acid are produced at rates higher than at the above undesired by-products.
Acetic acid is currently produced by catalytic carbonylation of methanol. The traditional catalyst system comprises a rhodium compound and a methyl halide promoter such as methyl iodide. Typically, the reaction is conducted at temperatures between 150xc2x0 C. and 200xc2x0 C. and pressures from 20-50 bar with the rhodium catalyst dissolved in the liquid reaction medium consisting mainly of acetic acid, water and methyl iodide. Under reaction conditions a number of interconversions between reactants and products proceeds, such as esterification and hydrolysis:
CH3OH+CH3COOHxe2x86x92CH3COOCH3+H2O
CH3I+H2Oxe2x86x92CH3OH+HI
2CH3OHxe2x86x92CH3OCH3+H2O
These reactions are, under reaction conditions, essentially governed by thermodynamic equilibrium. In accordance with the equations above the feedstock may consist of methanol, dimethyl ether+water, methyl acetate+water, or any mixture of methanol, dimethyl ether, methyl acetate and water.
The overall reaction takes place according to the equation:
CH3OH+COxe2x86x92CH3COOH (catalysts: Rh, CH3I)
Similar catalyst systems may be utilised in the synthesis of other carboxylic acids, e.g. propanoic acid by replacing methyl iodide promoter with ethyl iodide and replacing methanol with ethanol:
C2H5OH+COxe2x86x92C2H5COOH (catalysts: Rh, C2H5I)
Thus, by replacing the alcohol feed and the alkyl iodide promoter by their higher homologues, virtually any carboxylic acid may be produced in a similar way or more carboxylic acids may be produced simultaneously.
In industrial acetic synthesis, methanol feed is continuously fed into the reaction solution together with carbon monoxide. Under the prevailing reaction conditions the catalyst system also catalyses the water gas shift reaction whereby part of the carbon monoxide reacts with the water contained in the reaction solution to form hydrogen and carbon dioxide. Consequently, the reaction system will inevitably contain a finite concentration of hydrogen. Hydrogen will also typically be present in finite amounts in the carbon monoxide feed gas, which is most often produced from synthesis gas consisting of hydrogen and carbon monoxide, e.g. by cryogenic fractionation.
Although the carbonylation process is very selective, typically more than 99% based on methanol, small amounts of by-products being formed, especially propanoic acid, and organic carbonyl impurities like acetaldehyde and acetone, which tend to build up in the synthesis loop.
Primary carbonyl impurities such as acetaldehyde are particularly harmful, because they can react by self condensation to secondary aldol condensation products, such as e.g. croton aldehyde. These secondary aldol condensation products may further react with the iodide catalyst promoters to form organic iodides such as ethyl iodide, butyl iodide and hexyl iodide.
This problem is commonly recognised in the art and mentioned e.g. EP Patent Nos. 487,284, 768,295, 687,662 and U.S. Pat. No. 5,723,660 and references cited therein. In many of these by-products it is difficult to separate from the acetic acid product by conventional means such as distillation because of their boiling points being close to that of acetic acid or due to formation of azeotropes with acetic acid. Additionally, these by-products are known to act as poisons to the catalysts used in downstream processing of acetic acid to e.g. vinyl acetate.
Many attempts have been made to minimise the amount of these by-products in the final acetic acid product, for instance by treatment with ozone (U.S. Pat. No. 5,202,481, EP Patent No. 645,362), by contacting with silver-exchanged ion exchange resins (EP Patent No. 196,173), by contacting with activated carbon (WO 94/22,804), by complicated multiple fractionation of part of raw product stream (WO 98/17,619) in order to avoid excessive build-up of carbonyl impurities in the reaction loop. EP Patent No. 687,662 teaches that the concentration of acetaldehyde in the reaction medium be kept below 400 ppm. This is achieved by removing acetaldehyde, by distillation and subsequent extraction of the acetaldehyde with water, from the process liquid being recirculated to the carbonylation reactor.
In view of the above, a method is desirable that will reduce the formation of carbonyl impurities. One way of achieving this is by in situ catalytic hydrogenation of carbonyl impurities so as to transform e.g. acetaldehyde into ethanol and thereby maintain the acetaldehyde concentration at levels so low that the self-condensation reaction is significantly suppressed.
It is to be noted that, contrary to the carbonyl impurities, by-products propanoic acid (for which ethanol is a precursor) and any higher carboxylic acids may easily be separated from acetic acid by distillation because of the significant difference in boiling points and because propanoic acid does not form azeotropes with acetic acid. Moreover, propanoic acid is a valuable product with a number of industrial applications.
In conventional acetic acid synthesis it is common practice to produce the carbon monoxide feed by cryogenic fractionation of synthesis gas in order to achieve a low content of hydrogen in the feed because hydrogen tends to favour the formation of undesired by-products. Thus, EP Patent No. 728,727 teaches that the content of hydrogen in the carbon monoxide feed being formed in situ by the water gas shift reaction shall preferably be kept less than 2 bar in partial pressure as its presence may result in the formation of hydrogenation products.
The cryogenic separation of carbon monoxide from synthesis gas is a capital and energy intensive process. With less strict demands to the hydrogen content, it is possible to produce carbon monoxide feed more economically, either by carrying out the cryogenic separation to a lower degree of fractionation or by applying e.g. hollow fibre membranes which are commercially available, relatively inexpensive and easy to maintain and operate.
It has now been found that addition of ruthenium compounds to the carbonylation reaction solution conditions effectively reduces the formation of undesired carbonyl impurities whilst increasing the formation of ethanol, ethyl acetate and ethyl iodide being precursors for the formation of valuable propanoic acid.
One of the effects of adding ruthenium compounds to the reaction solution is that the amount of acetaldehyde in the carbonylation reactor is kept at low levels, such as less than 400 ppm.
Accordingly, the present invention provides a process for the carbonylation of methanol and/or reactive derivatives thereof which comprises contacting methanol and/or a reactive derivative thereof with carbon monoxide and hydrogen in the presence of at least (a) a rhodium catalyst, (b) a methyl halide and (c), a ruthenium compound as a hydrogenation catalyst.
When operating the invention the presence of hydrogen is advantageous because it reduces the amount of detrimental carbonyl impurities by converting these into valuable by-products.
The content of hydrogen in the carbon monoxide feed and generated in situ by the water gas shift reaction is preferably above 2 bar in partial pressure and, more preferably, above 3 bar in hydrogen partial pressure to obtain substantial reduction of formation of undesired by-products.
The following examples serve solely as an illustration of the invention: